High Frequency Absorptive Switch Architecture

ABSTRACT

An absorptive switch architecture suitable for use in high frequency RF applications. A switching circuit includes a common terminal and one or more ports, any of which may be selectively coupled to the common terminal by closing an associated path switch; non-selected, unused ports are isolated from the common terminal by opening an associated path switch. Between each path switch and a port are associated shunt switches for selectively coupling an associated signal path to circuit ground. Between each path switch and a port is an associated absorptive switch module. Each absorptive switch module includes a resistor coupled in parallel with a switch. The combination of the resistor and the switch of the absorptive switch module is placed in series with a corresponding signal path from each port to the common terminal, rather than in a shunt configuration.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No. 14/527,168 entitled “High Frequency Absorptive Switch Architecture”, Attorney Docket No. PER-129-PAP, filed on Oct. 29, 2014, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND (1) Technical Field

This invention generally relates to electronic signal switching devices, and more specifically to electronic signal switching devices having high frequency absorptive switch architectures.

(2) Background

Electronic signal switches are used in a wide variety of applications. One type of signal switch in common use is a field effect transistor (FET) that is actively controlled through a gate terminal to block or pass an electrical signal connected in series with the source and drain terminals of the FET. In many applications, the presence of a FET switch has a negligible effect on signals blocked or passed by the switch. However, in radio frequency (RF) circuits, the presence of a FET switch may have significant effects on the rest of the circuit, particularly with respect to termination impedance and isolation levels. Such effects arise because an “ON” (low impedance) FET has a non-zero resistance, and an “OFF” (high impedance) FET behaves as a capacitor.

For example, switch architectures for RF circuits typically have used one or more shunt termination FET switches in series with a fixed termination resistance to form a termination path to circuit ground for unused ports. In most cases, because of the imperfect switching characteristics of a shunt FET switch, an additional FET switch is needed to achieve high isolation of the termination path from the rest of the “OFF path” when the shunt FET is configured as a short to circuit ground.

FIG. 1A is a block diagram of an RF circuit 100 having a shunt switch architecture in accordance with the prior art. All of the switches in the RF circuit 100 may be implemented as FET switches, as described below in further detail. A common RF terminal RF_(C) may be selectively coupled to any of two or more RF pathways corresponding to RF terminals RF₁-RF_(N). For example, each of the RF terminals RF₁-RF_(N) may be coupled to respective radio antennas while RF_(C) is coupled to radio transceiver circuitry, such as in a cellular phone. In the illustrated example, to couple terminal RF₁ to RF_(C), a series-coupled RF₁ path switch 102 and an isolation switch 104 are set to “ON” (low impedance) to pass signals between RF₁ and RF_(C). Concurrently, one or more associated shunt switches 106 coupled from circuit ground to a signal path 105 between the RF₁ path switch 102 and the isolation switch 104 are set to “OFF” (high impedance). In addition, a termination switch 108 connected from circuit ground through a series termination resistor R_(Term1) to terminal RF₁ is also set to “OFF”.

When terminal RF₁ is coupled to RF_(C) as described above, signals pass between RF₁ and RF_(C) through the resistance provided by the RF₁ path switch 102 and the isolation switch 104, and neither signal path 105 nor RF₁ are coupled to circuit ground through the shunt switches 106 or the termination switch 108, respectively. In order to isolate RF_(C) from the other terminals RF₂-RF_(N) (all of which have signal paths similar to the RF₁ signal path), the other RF pathways are set to be effectively isolated from RF_(C). When another RF terminal is to be coupled to RF_(C), then the RF₁ signal path must be similarly effectively isolated from RF_(C). This is accomplished by setting RF₁ path switch 102 to “OFF” (high impedance). However, various types of switches—including FET switches—exhibit current leakage when nominally “OFF”. In such embodiments, the shunt switches 106 are provided. Thus, when the RF₁ circuit path is to be isolated, the corresponding shunt switches 106 are set to “ON” (low impedance) in order to shunt leakage current through the RF₁ path switch 102 to ground.

In order to provide a fixed termination impedance when RF₁ is “OFF”, the termination switch 108 is set to “ON” (low impedance) in order to couple RF₁ though R_(Term1) to circuit ground. The value of R_(Term1) is application dependent, but is generally set such that, taking into account the “ON” resistance (R_(on)) of the termination switch 108 itself, the characteristic impedance at RF₁ is about 50 ohms for most RF applications. Without such termination, a nominally “OFF” antenna may reflect received power back into the RF circuit 100 and cause signal interference with other RF circuitry, such as another antenna that is “ON”.

In order to achieve high isolation from RF₁ to RF_(C), and to raise the level of impedance that is effectively in parallel with R_(Term1) and termination switch 108, the isolation switch 104 is set to “OFF” (high impedance). In a typical application, the “OFF” resistance of the isolation switch 104 is set at thousands of ohms in order to achieve 20-30 dB of isolation.

As noted above, all of the switches in the RF circuit 100 of FIG. 1A may be implemented as FET switches. FIG. 1B is a schematic diagram of a typical switch configuration 120 suitable for use in the circuit shown in FIG. 1A. Shown are one or more FETs series stacked (for voltage handling) in a conventional configuration. A drain D to source S resistance R of high value ensures that each switch provides uniform resistance when the FETs are set to “OFF” (high impedance) under the control of a signal to the gate structures G; the value of R is application dependent. When the FETs are set to “ON” (low impedance) under the control of a signal to the gate structures G, the resistance R_(on) from the drain D and source S is quite low but not zero.

A problem with the shunt switch architecture shown in FIG. 1A is that, as the frequency of operation of the RF circuit 100 increases, the combined parasitic FET capacitance of the isolation switch 104 and termination switch 108 respectively begin to degrade both the isolation level and the termination impedance of the RF circuit 100 as a whole. To counter this behavior, the FET termination switch 108 is often made smaller to minimize its capacitance. This in turn means that more of the RF power from the nominally isolated RF₁-RF_(N) terminals is terminated in the corresponding termination switches 108 rather than in the corresponding termination resistors R_(Term1)-R_(TermN) because of the higher resistance of the smaller FET switch devices. Terminating such power in the smaller FET switches can lead to premature failure; to avoid that issue, the power handling capability for the RF circuit 100 would have to be specified at a reduced level, which may be commercially disadvantageous.

Accordingly, there is a need for a switch architecture suitable for use with high frequency RF signals that does not exhibit the problems of the prior art. The present invention meets this need.

SUMMARY OF THE INVENTION

Aspects of the invention include an absorptive switch architecture suitable for use in high frequency RF applications. A switching circuit includes a common terminal RF_(C) and one or more ports RF₁-RF_(N), any of which may be selectively coupled to the common terminal RF_(C) by closing an associated path switch; non-selected, unused ports are isolated from the common terminal RF_(C) by opening an associated path switch. Between each path switch and a port RF₁-RF_(N) are associated shunt switches for selectively coupling an associated signal path to circuit ground.

Between each path switch and a port RF₁-RF_(N) is an associated absorptive switch module. Each absorptive switch module includes a resistor R_(Term) coupled in parallel with a switch. Accordingly, the combination of the resistor R_(Term) and the switch of the absorptive switch module is placed in series with a corresponding signal path from each RF input port to the common terminal RF_(C), rather than in a shunt configuration.

In operation, when RF_(C) is to be coupled to (for example) terminal RF₁ (i.e., an “ON” state), the RF₁ path switch is set to “ON” (low impedance) and the associated shunt switches are set to “OFF”. In addition, the switch of the absorptive switch module is set to “ON”, allowing signal transmission between RF_(C) and RF₁. In this mode of operation, the parallel combination of the switch resistance R_(on) and the resistor R_(Term) looks like two resistors in parallel: R_(on)∥R_(Term). For RF applications, since insertion loss is critical, R_(on) is set to be much less than the system characteristic impedance.

In the converse state, when terminal RF₁ is to be isolated from RF_(C) (i.e., an “OFF” state), the RF₁ path switch is set to “OFF” (high impedance) and the associated shunt switches are set to “ON”. In addition, the switch of the absorptive switch module is set to “OFF”. In this mode of operation, the switch has the characteristics of a capacitor (with value C_(off)) rather than a resistor (with value R_(on)). Thus, the parallel combination of the switch capacitance C_(off) and the resistor R_(Term) looks like a parallel RC circuit: C_(off)∥R_(Term). Notably, the associated shunt switches are partially repurposed to shunt any RF signal present on the RF₁ terminal to ground through the absorptive switch module.

There are at least four benefits to the absorptive switch architecture described above over traditional circuit configurations:

First, a second isolating device is no longer needed to isolate an RF_(N) terminal from the rest of the “OFF” path to RF_(C). Accordingly, the number of distinct switch elements is reduced from two to one.

Second, the parallel combination of the termination resistor R_(Term) and the resistance R_(on) of the switch begins to look more capacitive as frequency is increased (i.e., the capacitive reactance increases). This is a beneficial behavior because the impedance to circuit ground of the shunt switches begins to look more inductive (i.e., inductive reactance increases) as frequency is increased. These two reactive impedances (i.e., reactances), when added in series, substantially cancel each other and the result remains more nearly a real impedance close to a targeted characteristic impedance.

Third, terminated RF power can be more consistently and completely terminated in the R_(Term) resistor and not in the switch of the absorptive switch module, and power is also dissipated across the parallel shunt switches connected in series with the absorptive switch module.

Fourth, the parallel combination of the R_(Term) termination resistor and the switch is very modular in nature, particularly when the switch is implemented as a FET.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of an RF circuit having a shunt switch architecture in accordance with the prior art.

FIG. 1B is a schematic diagram of a typical switch configuration suitable for use in the circuit shown in FIG. 1A.

FIG. 2A is a block diagram of an RF circuit having an absorptive switch architecture suitable for use in high frequency RF applications.

FIG. 2B is a schematic diagram of one absorptive switch module (ASM) configuration suitable for use in the circuit shown in FIG. 2A.

FIG. 3A is a schematic diagram of an impedance circuit model corresponding to the ASM and the shunt switches shown in FIG. 2A.

FIG. 3B is graph of the real and imaginary impedances for the impedance circuit model shown in FIG. 3A, and of the corresponding ratio of derivative values, dZimag/dFreq.

FIG. 4A is a Smith chart comparing the inductive termination impedance (increasing with frequency) of the prior art termination switch (108) plus termination resistor R_(Term1) of FIG. 1A against the combined (capacitive and inductive) termination impedance of a model circuit of the type shown by block 202 in FIG. 3A, as seen at the “OFF” RF_(N) port.

FIG. 4B is a Smith chart separately showing the inductive impedance behavior of the shunt switches (106′) and the capacitive impedance behavior of the ASM (202), both of FIG. 3A.

FIG. 5A shows real impedance vs. freq. for the ASM and the associated shunt switches of one embodiment of the absorptive switch architecture, and their summation.

FIG. 5B shows imaginary impedance vs. freq. for the ASM and the associated shunt switches of one embodiment of the absorptive switch architecture, and their summation.

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2A is a block diagram of an RF circuit 200 having an absorptive switch architecture suitable for use in high frequency RF applications. While similar in some aspects to the circuit shown in FIG. 1A, a critical distinction is that the isolation switch 104 and termination switch 108 of FIG. 1A have been replaced entirely by an absorptive switch module 202.

FIG. 2B is a schematic diagram of one absorptive switch module 202 configuration suitable for use in the circuit shown in FIG. 2A. As illustrated, the absorptive switch module 202 includes a resistor R_(Term) coupled in parallel with a switch 204, shown in the illustrated embodiment as a FET switch; however, other switching devices may be used for the switch 204, such as switches implemented as micro-electromechanical systems (MEMS), since nearly every practical switching component exhibits some level of “OFF” capacitance. While only one switch 204 is shown, one or more series stacked switches may be used for handling applied voltages, in known fashion.

For most uses, particularly RF applications, the characteristic impedance of R_(Term) may be set to about 50 ohms; however, the concepts described in this document work for a broad range of characteristic impedances. The parallel combination of the resistor R_(Term) and the switch 204 of the absorptive switch module 202 is placed in series with the signal path from each RF input to RF_(C), rather than in a shunt configuration.

In operation, when RF_(C) is to be coupled to terminal RF₁ (i.e., an “ON” state), the RF₁ path switch 102 is set to “ON” (low impedance) and the associated shunt switches 106 are set to “OFF”. In addition, the switch 204 of the absorptive switch module 202 is set to “ON”, allowing signal transmission between RF_(C) and RF₁. In this mode of operation, the parallel combination of the switch 204 resistance R_(on) and the resistor R_(Term) looks like two resistors in parallel: R_(on)∥R_(Term). For RF applications, since insertion loss is critical, R_(on) is set to be much less than the system characteristic impedance (which is 50 ohms for all examples here; in other applications, a different system characteristic impedance may be used). For example, R_(on) may be set to be on the order of a few ohms for many RF applications. Therefore, the parallel combination R_(on)∥R_(Term) looks like a resistance close to R_(on) over a broad frequency range.

In the converse state, when terminal RF₁ is to be isolated from RF_(C) (i.e., an “OFF” state), the RF₁ path switch 102 is set to “OFF” (high impedance) and the associated shunt switches 106 are set to “ON”. In addition, the switch 204 of the absorptive switch module 202 is set to “OFF”. In this mode of operation, the switch 204 has the characteristics of a capacitor (with value C_(off)) rather than a resistor (with value R_(on)). Thus, the parallel combination of the switch 204 capacitance C_(off) and the resistor R_(Term) looks like a parallel RC circuit: C_(off)∥R_(Term). In a typical FET implementation of the switch 204 for RF applications, C_(off) would be on the order of femtofarads. For example, assuming a FET process figure of merit R_(on)*C_(off) of 200 fs, if R_(on) is set to about 3 ohms, then C_(off) is about 67 femtofarads.

Notably, the associated shunt switches 106 are partially repurposed to shunt any RF signal present on the RF₁ terminal to ground through the absorptive switch module 202.

While FIG. 2A shows multiple path switches RF₁-RF_(N), each of which may have a corresponding absorptive switch module 202, other embodiments may include a single path switch, such as in a single-pole, single-throw (SPST) high isolation switch application. In an SPST embodiment, an absorptive switch module 202 may be coupled to either or both terminals of the switch. Further, in general, an absorptive switch module 202 may be coupled to any port or element where termination would be useful, but need not be coupled to every port. Thus, in some embodiments, one or more ports may be coupled to an absorptive switch module 202, while other ports may be coupled to a conventional termination circuit and/or be unterminated (i.e., reflective).

To better understand the benefits of the absorptive switch architecture, FIG. 3A is a schematic diagram of an impedance circuit model 300 corresponding to the absorptive switch module 202 and the associated shunt switches 106 shown in FIG. 2A. The associated shunt switches 106 are essentially the components on the “P1” side of node 210 in FIG. 2A (ignoring the RF path switch 102, which is in an “OFF” or high impedance state for purposes of this example). The switch 204 and the resistor R_(Term) of the absorptive switch module 202 are the components on the “P2” side of node 210 in FIG. 2A.

The components of the absorptive switch module 202 when in the “OFF” (high impedance) state have an impedance Z_(Term)=(Ra, −jXa), where “Ra” is essentially the resistance of an instance of the resistor R_(Term) and “Xa” is essentially the capacitive reactance of an instance of the switch 204 in the “OFF” state (typically there are parasitic resistances and capacitances that modify these levels slightly). When “ON” (low impedance), the parallel shunt switches 106 essentially form a distributed inductance-resistance (LR) network 106′, as shown in FIG. 3A. That is, each shunt switch 106 may be modeled has having a resistance R and an inductance L₁; in addition, other component or parasitic inductances L₂ may be present in particular embodiments. The components of the associated shunt switches 106′ have an impedance Z_(LR)=(Rb, +jXb), where “Rb” is the effective resistance of the combined associated shunt switches 106 and “Xb” is the inductive reactance of the combined structure of the distributed interconnect between the shunt switches 106′ and ground inductance associated with the shunt switches 106′ in the “ON” state.

For the circuit model 300 as shown, the total impendence looking into the RF_(N) port connected to the serially-connected absorptive switch module 202 and the distributed shunt switch LR network 106′ is given by Z_(in)=Z_(Term)+Z_(LR); accordingly, Z_(in)=(Ra, −j/Xa)+(Rb, +jXb), which may be rearranged as Z_(in)=((Ra+Rb), j(Xb−Xa)). Thus, the negative capacitive reactance of each switch 204 behaves as a negative inductance that substantially offsets the positive inductive reactance of the associated shunt switches 106; this holds valid over a wide range of frequencies (e.g., about 1-40 GHz), until the capacitive impedance of the switch 204 becomes comparable in magnitude to R_(Term). More particularly, the useful range of frequencies is in the region where the derivative of the imaginary component of the inductance, dZimag, divided by the derivative of the frequency, dFreq, is negative. This can be seen in FIG. 3B, which is a graph of the real impedance 310 (also labeled Zt_res) and the imaginary impedance 312 (also labeled Zt_img) for the absorptive switch module (202) shown in FIG. 3A, and of the corresponding ratio of derivative values 314, dZimag/dFreq (also labeled dZi/dF); in the illustrated graph, dZimag/dFreq 314 is negative below about 100 GHz, and is particularly useful when below the indicated 40 GHz dotted line, where the slope approaches about −1 jOhm/GHz.

For many RF applications, Z_(in) is ideally equal to (50, +j0) ohms (i.e., essentially purely resistive). In terms of design considerations for such RF applications, the values selected for all of the components (taking into account parasitic RLC values) should be such that (Ra+Rb) is closer to 50 ohms than just Ra alone, and such that j(Xb−Xa) is closer to j0 than just −jXa alone.

It should be appreciated that the disclosed absorptive switch architecture leverages the inherently capacitive nature of the parallel “OFF” switch 204 of FIG. 2B to provide a termination impedance that is capacitive at higher frequencies and which accordingly substantially offsets the inductive impedance behavior of the shunt switches 106′ at higher frequencies. This is in stark contrast to conventional designs that provide a termination impedance that is always inductive at higher frequencies. For example, FIG. 4A is a Smith chart 400 comparing the inductive termination impedance 402 (i.e., inductive reactance increasing with frequency) of the prior art termination switch 108 plus termination resistor R_(Term1) of FIG. 1A against the combined (capacitive and inductive reactance) termination impedance 404 of a model circuit of the type shown by block 202 in FIG. 3A and described above, as seen at the “OFF” RF_(N) port. The frequency range for this example is about 1-40 GHz.

FIG. 4B is a Smith chart 410 separately showing an increase in the inductive reactance of the shunt switches 106′ (curve 412) and the capacitive reactance of the absorptive switch module 202 (curve 414), both of FIG. 3A; the respective arrows of the curves 412, 414 indicate off-setting changes in impedance as frequency increases. The frequency range for this example is about 1-40 GHz.

Accordingly, the capacitive behavior of the parallel “OFF” switch 204 of the absorptive switch module 202 can be balanced against the distributed inductive behavior of the “ON” parallel shunt switches 106 so as to provide a near ideal terminated port impedance of 50 ohms for RF applications (of course, other impedance levels may be selected for particular applications by changing component values). This results in a very good voltage standing wave ratio (VSWR) versus frequency, as illustrated for a particular model implementation of the absorptive switch architecture in FIG. 5A and FIG. 5B.

FIG. 5A is a graph of real impedance (resistance) versus frequency for the absorptive switch module 202 and the associated shunt switches 106 of one embodiment of the absorptive switch architecture, and their summation. Graph line 502 represents the frequency dependent real impedance Z1 re of the active components on the “P1” side of node 210 in FIG. 2A (i.e., the associated shunt switches 106). As illustrated, Z1 re increases with frequency. Graph line 504 represents the frequency dependent real impedance Z2 re of the components on the “P2” side of node 210 in FIG. 2A (i.e., the absorptive switch module 202). As illustrated, Z2 re decreases with frequency. Graph line 506 represents the sum Zre of Z1 re and Z2 re. As can be seen, the total real impedance Zre is near 50 ohms over a wide frequency range, since much of the frequency-dependent changes to the two constituent real impedances Z1 re and Z2 re substantially offset each other.

FIG. 5B is a graph of imaginary impedance (reactance) versus frequency for the absorptive switch module 202 and the associated shunt switches 106 of one embodiment of the absorptive switch architecture, and their summation. Graph line 512 represents the frequency dependent imaginary impedance Z1 i of the active components on the “P1” side of node 210 in FIG. 2A (i.e., the associated shunt switches 106). As illustrated, Z1 i is positive and its absolute magnitude increases with frequency (i.e., the impedance becomes increasingly more positive, and thus more inductive). Graph line 514 represents the frequency dependent imaginary impedance Z2 i of the components on the “P2” side of node 210 in FIG. 2A (i.e., the absorptive switch module 202). As illustrated, Z2 i is negative and its absolute magnitude increases with frequency (i.e., the impedance becomes increasingly more negative, and thus more capacitive). Graph line 516 represents the sum Zi of Z1 i and Z2 i. As can be seen, the total imaginary impedance Zi is near zero ohms over a wide frequency range, since much of the frequency-dependent changes to the two constituent imaginary impedances Z1 i and Z2 i substantially offset each other.

There are at least four benefits to the absorptive switch architecture described above over traditional circuit configurations:

First, a second isolating device (i.e., isolation switch 104) is no longer needed to isolate an RF_(N) terminal from the rest of the “OFF” path to RF_(C). Instead, the disclosed architecture utilizes the “ON” (low impedance) path to circuit ground of the partially repurposed associated shunt switches 106 as the RF ground for the RF_(N) termination impedance. Accordingly, the number of distinct switch elements is reduced from two (isolation switch 104 and termination switch 108) to one (the switch 204 in the absorptive switch module 202).

Second, the parallel combination of the termination resistor R_(Term) and the resistance R_(on) of the switch 204 begins to look more capacitive as frequency is increased. This is a beneficial behavior because the impedance to circuit ground of the shunt switches 106 begins to look more inductive as frequency is increased. These two reactive impedances, when added in series, substantially cancel each other and the result remains more nearly a real impedance close to the targeted characteristic impedance, as shown in FIG. 5A and FIG. 5B. This results in excellent isolation, generally exceeding −28 dB at 40 GHz and reaching in excess of −35 dB at 40 GHz for one model circuit. The insertion loss of the isolation mode of several modeled circuits was less than −4 dB from about 1-40 GHz. The return loss of one modeled circuit exceeded −22 dB from about 1-40 GHz and reached in excess of −30 dB at certain frequencies. In one modeled circuit, the absorptive switch architecture had a return loss significantly better than a comparable circuit implemented with a conventional termination architecture.

Third, terminated RF power can be more consistently and completely terminated in the R_(Term) resistor and not in the switch 204 of the absorptive switch module 202 (see FIG. 2B). The R_(Term) resistor can be sized to the required power level, and the parallel switch 204 can be “stacked” to achieve the associated voltage handling capability. Modeling example embodiments of the absorptive switch architecture shows that 60% to 90% of incident power is dissipated by the R_(Term) resistor from about 1-40 GHZ (the percentage varies with frequency), with the balance dissipated through the parallel shunt switches 106.

Fourth, the parallel combination of the R_(Term) termination resistor and the switch 204 is very modular and compact in nature, particularly when the switch 204 is implemented as a FET. This makes the use of the absorptive switch module 202 and variants of that module very straightforward. For example, multiple absorptive switch modules 202 can be series connected to handle higher power, and the compact size of such modules provides flexibility in their placement within integrated circuit layouts. Further, because of the distributed nature of the shunt switches 106 (see FIG. 2A), absorptive switch modules 202 can be readily added, allowing changing a circuit design from a reflective switch to a terminated switch very easily. In addition, each absorptive switch module 202 requires only a single high impedance gate drive connection for control.

Referring back to FIG. 2B, also shown is an optional body diode 206 coupled between the body of the FET switch 204 and the source S. A body diode 206 may be useful in some implementations to reduce accumulated charge, in accordance with the teachings of U.S. Pat. No. 7,910,993, entitled “Method and Apparatus for Use in Improving Linearity of MOSFETS Using an Accumulated Charge Sink”, issued on Mar. 22, 2011 and assigned to the assignee of the present invention, the contents of which are hereby incorporated by reference.

Another aspect of the invention includes a method for terminating one or more unused ports of a switching circuit, including coupling at least one unused port to an absorptive switch module. In some embodiments, the absorptive switch module includes a switch and a resistor coupled in parallel, wherein the switch has the characteristics of a resistor in a first (closed) state and of a capacitor in a second (open) state.

Yet another aspect of the invention includes a method for terminating at least one unused port of a switching circuit, including the steps of:

STEP 1: providing a switching circuit including:

-   -   a. a common terminal;     -   b. at least one signal path, wherein at least one such signal         path includes:         -   i. a path switch connected in series with the common             terminal and the signal path, for selectively coupling the             signal path to the common terminal;         -   ii. at least one shunt switch connected between the signal             path and circuit ground, for selectively coupling the signal             path to circuit ground; and         -   iii. an absorptive switch module coupled in series with the             path switch and a port, the absorptive switch module             including a switch and a resistor coupled in parallel,             wherein the switch has the characteristics of a resistor in             a first state and of a capacitor in a second state;

STEP 2: selectively isolating a signal path from the common terminal by:

-   -   a. selecting a state for the path switch that decouples the         signal path from the common terminal;     -   b. selecting a state for each shunt switch that couples the         signal path to circuit ground; and     -   c. selecting a state for the absorptive switch module in which         the switch is set to a state that causes the absorptive switch         module to behave as a parallel resistor-capacitor network.

As should be readily apparent to one of ordinary skill in the art, various embodiments of the invention can be implemented to meet a wide variety of specifications. Thus, selection of suitable component values are a matter of design choice (so long as the frequencies of interest mentioned above can be handled). The switching and passive elements may be implemented in any suitable integrated circuit (IC) technology, including but not limited to MOSFET and IGFET structures. Integrated circuit embodiments may be fabricated using any suitable substrates and processes, including but not limited to standard bulk silicon, silicon-on-insulator (SOI), silicon-on-sapphire (SOS), GaAs pHEMT, and MESFET processes. Voltage levels may be adjusted or voltage polarities reversed depending on a particular specification and/or implementing technology (e.g., NMOS, PMOS, or CMOS). Component voltage, current, and power handling capabilities may be adapted as needed, for example, by adjusting device sizes, “stacking” components to handle greater voltages, and/or using multiple components in parallel to handle greater currents.

A number of embodiments of the invention have been described. It is to be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, some of the steps described above may be order independent, and thus can be performed in an order different from that described. It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the invention, which is defined by the scope of the following claims, and that other embodiments are within the scope of the claims. 

What is claimed is:
 1. A switching circuit having at least one port coupled to an absorptive switch module in series with at least an RF common terminal, wherein the absorptive switch module has a first relatively small resistor in parallel with a second relatively large resistor in a first state and of the second relatively large resistor in parallel with a capacitor in a second state.
 2. The switching circuit of claim 1, wherein the first relatively small resistor has a resistance equal to an “ON” FET resistance, the capacitor has a capacitance equal to C_(off) of an “OFF” FET, and the second relatively large resistor has a resistance equal to a characteristic impedance of the switching circuit.
 3. The switching circuit of claim 2, further including at least one shunt switch having characteristics of a distributed inductance-resistance (LR) network coupled from circuit ground to a signal path in an “ON” state and having characteristics of a distributed inductance-capacitance (LC) network coupled from circuit ground to the signal path in a second state, wherein the impedance of the distributed LR network is Z_(LR)=(Rb +jXb), where Rb and +jXb are the series equivalent resistance and reactance of the at least one shunt switch in the “ON” state and the distributed interconnect between the shunt switches and ground, and wherein the impedance of the absorptive switch module when in the “OFF” state is Z_(Term)=(Ra −jXa), where Ra and −jXa are series equivalent resistance and reactance of the parallel combination of the relatively large resistor and C_(off).
 4. The switching circuit of claim 3, wherein Ra+Rb is essentially equal to the characteristic impedance of the switching circuit.
 5. The switching circuit of claim 4, wherein the characteristic impedance is 50 ohms.
 6. The switching circuit of claim 3, wherein Z_(term)+Z_(LR)=Z₀, and wherein Z₀ is the characteristic impedance of the switching circuit.
 7. The switching circuit of claim 2, wherein the absorptive switch module comprises a FET having a body and a source, and the absorptive switch module further includes a body diode coupled between body and the source of the FET.
 8. A switching circuit including: (a) a common terminal; (b) at least one port; (c) an absorptive switch module having a first and second terminal, the first terminal coupled to a port, the absorptive switch module including a first switch and a resistor coupled in parallel between the first and second terminal; (d) a path switch having a first terminal coupled to the common port and a second terminal coupled to the second terminal of the absorptive switch module; and (e) at least one shunt switch having a first terminal coupled to the second terminal of the path switch and a second terminal coupled to ground, wherein the absorptive switch module has an impedance equal to a termination resistance in a first state and equal to a relatively small resistance, Ron, in parallel with the termination resistance in a second state.
 9. The switching circuit of claim 8, wherein the first switch is a field effect transistor (FET).
 10. The switching circuit of claim 9, wherein the termination resistance has a first terminal coupled to a source terminal of the FET and a second terminal coupled to a drain of the FET.
 11. The switching circuit of claim 9, wherein the impedance of the absorptive switch module is equal to the termination resistance in parallel with a capacitance Coff in the first state.
 12. The switching circuit of claim 11, wherein the impedance from the absorptive switch module to ground through the at least one shunt switch, with the at least one shunt switch in an “ON” state, is inductive and matched to C_(off).
 13. The switching circuit of claim 12, further including: (a) at least a second port; (b) at least a second absorptive switch module having a first and second terminal, the first terminal coupled to the second port; (c) at least a second path switch having a first terminal coupled to the common terminal and a second terminal coupled to the second terminal of the second absorptive switch module; and (d) at least a second shunt switch having a first terminal coupled to the second terminal of the path switch and a second terminal coupled to ground, wherein the second absorptive switch module has an impedance equal to a second termination resistance in parallel with a capacitance C_(off2) in the first state of the second absorptive switch module; and wherein the impedance from the second absorptive switch module to ground through the second shunt switch, with the second shunt switch in an “ON” state, is inductive and matched to C_(off2).
 14. The switching circuit of claim 8, wherein the termination resistance has an impedance of approximately 50 Ohms.
 15. The switching circuit of claim 8, wherein the impedance from the absorptive switch module to ground through the at least one shunt switch, with the shunt switch in an “ON” state, is inductive and matched to C_(off) for frequencies from 1 to 40 GHz.
 16. The switching circuit of claim 15, wherein C_(off) has an impedance of approximately 67 femtofarads.
 17. The switching circuit of claim 15, wherein the resistance R_(on) is approximately 3 Ohms.
 18. The switching circuit of claim 15, wherein the first terminal of the shunt switch is a source of FET and the second terminal of a drain of the FET.
 19. The switching circuit of claim 8, wherein the shunt switch is a FET.
 20. The switching circuit of claim 8, wherein the switching circuit is a radio frequency (RF) switching circuit.
 21. The switching circuit of claim 8, wherein the absorptive switch module has a capacitive reactance that increases with increasing frequency, and wherein the conductive path has an inductive reactance from the absorptive switch module to ground through the shunt switch that increases with increasing frequency.
 22. The switching circuit of claim 21, wherein the increase in the capacitive reactance of the absorptive switch is essentially equal to the increase in the inductive reactance of the path through the shunt switch with increasing frequency.
 23. The switching circuit of claim 21, wherein the impedance of the absorptive switch module functions as a negative reactance that substantially offsets a positive reactance of the at least one shunt switch for signal frequencies within a range of about 1 to 40 GHz.
 24. An RF switching circuit embodied in an integrated circuit, including: (a) a common terminal; (b) at first RF port; (c) a path switch means having a first and second terminal, the first terminal coupled to the common terminal for imposing a relatively high resistance to RF signals in a first state and for imposing a relatively low resistance to RF signals in a second state; (d) an absorptive switch module means having a first terminal coupled to the second terminal of the path switch means and a second terminal coupled to the first RF port, the absorptive switch module means for imposing a resistance R_(term) in parallel with a capacitance, C_(off) in a first state and imposing a resistance R_(on) in parallel with the resistance R_(term) in a second state; and (e) a shunt switch means having a first terminal coupled to the second terminal of the path switch means and having a second terminal coupled to ground, for providing a low resistance path to ground in a first state and a high resistance path to ground in a second state.
 25. The RF switching circuit of claim 24, wherein the path switch means is in the first state when the absorptive switch module means is in the second state, thereby creating a relatively low impedance signal path from the common terminal to the first RF port.
 26. The RF switching circuit of claim 25, wherein the path switch means is in the second state when the absorptive switch module means is in the first state, thereby terminating the first RF port in a resistance equal to R_(term).
 27. The RF switching circuit of claim 26, wherein the shunt switch means is in the first state when the path switch means is in the second state.
 28. The RF switching circuit of claim 27, wherein the shunt switch means is an FET.
 29. The RF switching circuit of claim 27, wherein a conductive path from the first terminal of the absorptive switch module means to ground through the shunt switch means with the shunt switch means in the first state has an impedance that is inductive.
 30. The RF switching circuit of claim 29, wherein the conductive path from the first RF port to the second terminal of the absorptive switch module means has an impedance that is capacitive and an inductive reactance of the conductive path from the first terminal of the absorptive switch module means to ground through the shunt switch means with the shunt switch means in the first state is equal to the capacitive reactance of the conductive path from the first RF port to the second terminal of the absorptive switch module means, such that the impedance from the first RF port to ground through the absorptive switch module means in the first state and the shunt switch means in the first state is essentially purely resistive.
 31. The RF switch circuit of claim 30, further including: (a) a second port; (b) a second path switch means having a first and second terminal, the first terminal coupled to the common terminal for imposing a relatively high resistance to RF signals in a first state and for imposing a relatively low resistance to RF signals in a second state; (c) at least a second absorptive switch module means having a first terminal coupled to the second terminal of the second path switch means and a second terminal coupled to the second RF port, the second absorptive switch module means for imposing a resistance R_(term) in parallel with a capacitance, C_(off2) in a first state and imposing a resistance R_(on2) in parallel with the resistance R_(term) in a second state.
 32. The RF switch circuit of claim 31, wherein the second path switch means is in the first state when the second absorptive switch module means is in the second state, thereby creating a relatively low impedance signal path from the common terminal to the second RF port.
 33. The RF switching circuit of claim 32, wherein the second path switch means is in the second state when the second absorptive switch module means is in the first state, thereby terminating the second RF port in a resistance equal to R_(term.)
 34. The RF switching circuit of claim 33, wherein the second shunt switch means is in the first state when the second path switch means is in the second state.
 35. The RF switching circuit of claim 34, wherein the conductive path from the first terminal of the second absorptive switch module means to ground through the second shunt switch means with the second shunt switch means in the first state has an impedance that is inductive.
 36. The RF switching circuit of claim 35, wherein a conductive path from the second RF port to the second terminal of the second absorptive switch module means has an impedance that is capacitive and an inductive reactance of the conductive path from the first terminal of the second absorptive switch module means to ground through the second shunt switch means with the second shunt switch means in the first state is equal to the capacitive reactance of the conductive path from the second RF port to the second terminal of the second absorptive switch module means, such that the impedance from the second RF port to ground through the second absorptive switch module means in the first state and the second shunt switch means in the first state is essentially purely resistive.
 37. The RF switching circuit of claim 30, wherein the impedance from the first RF port to ground through the absorptive switch module means in the first state and the shunt switch means in the first state is essentially equal to a characteristic impedance of the RF switching circuit.
 38. The RF switching circuit of claim 37, wherein an insertion loss at the first RF port looking into the port from outside the RF switching circuit is less than −4 dB from about 1 GHz to 40 GHz at the characteristic impedance.
 39. The RF switching circuit of claim 38, wherein the first switch means is a FET.
 40. The RF switching circuit of claim 30, wherein the absorptive switch module means includes: (a) a first switch means; (b) a termination resistance coupled in parallel with the first switch mean.
 41. A method for connecting a common terminal with at least one port, including: (a) providing a common terminal; (b) providing at least one port; (c) providing a path switch having a first terminal and second terminal; (d) providing a shunt switch having a first and second terminal; (e) coupling the first terminal of the path switch to the common port; (f) coupling the first terminal of the shunt switch to the second terminal of the port switch; (g) coupling the second terminal of the shunt switch to ground; (h) providing an absorptive switch module having a first terminal and a second terminal, the absorptive switch module including a first switch in parallel with a termination resistor coupled between the first and second terminal; (i) coupling the first terminal of the absorptive switch to the second terminal of the path switch; (j) coupling the second terminal of the first switch to the port; (k) turning on the path switch; (l) turning on the first switch within the absorptive switch module; and (m) turning off the shunt switch.
 42. The method of claim 41, further for isolating the port from the common terminal and terminating the port in a characteristic impedance, further including; (a) turning off the path switch; (b) turning off the first switch within the absorptive switch module; and (c) turning on the shunt switch.
 43. The method of claim 42, further including matching an off-capacitance, C_(off) of the first switch to an inductance of the conductive path from the first terminal of the first switch to ground through the shunt switch with the shunt switch on and with the first switch off. 